Method for processing a lithographic printing plate

ABSTRACT

A method for processing a lithographic printing plate includes development with an alkaline solution and gumming with a first gum solution and subsequently with a second gum solution, wherein both gum solutions are provided in a cascade configuration whereby the second gum solution overflows into the first gum solution and wherein the gum solution(s) include a nitrate salt.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a 371 National Stage Application ofPCT/EP2017/052417, filed Feb. 3, 2017. This application claims thebenefit of European Application No. 16160576.1, filed Mar. 16, 2016,European Application No. 16160591.0, filed on Mar. 16, 2016, EuropeanApplication No. 16160616.5, filed on Mar. 16, 2016. European ApplicationNo. 16160627.2, filed on Mar. 16, 2016 and European Application No.16168969.0, filed on May 10, 2016, which are incorporated by referenceherein in their entirety.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a method for processing lithographicprinting plates with a reduced consumption of processing liquids.

2. Description of the Related Art

Lithographic printing typically involves the use of a so-called printingmaster such as a printing plate which is mounted on a cylinder of arotary printing press. The master carries a lithographic image on itssurface and a print is obtained by applying ink to said image and thentransferring the ink from the master onto a receiver material, which istypically paper. In conventional lithographic printing, ink as well asan aqueous fountain solution (also called dampening liquid) are suppliedto the lithographic image which consists of oleophilic (or hydrophobic,i.e. ink-accepting, water-repelling) areas as well as hydrophilic (oroleophobic, i.e. water-accepting, ink-repelling) areas. These areas canalso be referred to as printing and non-printing areas respectively oras image and non-image areas respectively. In so-called driographicprinting, the lithographic image consists of ink-accepting andink-abhesive (ink-repelling) areas and during driographic printing, onlyink is supplied to the master.

Lithographic printing masters are generally obtained by the image-wiseexposure and processing of a printing plate precursor (referred tohereafter as “plate material” or briefly as “plate”), which contains aheat- or light-sensitive coating on a substrate. The coating of theplate material is exposed image-wise to heat or light, typically bymeans of a digitally modulated exposure device such as a laser, whichtriggers a (physico-)chemical process, such as ablation, polymerization,insolubilization by cross-linking of a polymer or by particlecoagulation of a thermoplastic polymer latex, solubilization by thedestruction of intermolecular interactions or by increasing thepenetrability of a development barrier layer. Although some platematerials are capable of producing a lithographic image immediatelyafter exposure, the most popular plate materials require wet processingwith a developer since the exposure produces a difference of solubilityor of rate of dissolution in a developer between the exposed and thenon-exposed areas of the coating. In positive-working plate materials,the exposed areas of the coating dissolve in the developer while thenon-exposed areas remain resistant to the developer. In negative-workingplate materials, the non-exposed areas of the coating dissolve in thedeveloper while the exposed areas remain resistant to the developer.Most plate materials contain a hydrophobic coating on a hydrophilicsubstrate, so that the areas which remain resistant to the developerdefine the ink-accepting, printing areas of the plate while thehydrophilic substrate is revealed by the dissolution of the coating inthe developer at the non-printing areas.

Conventionally, a plate material is developed by immersing it in, orspraying it with a developer as it passes through the processingapparatus. Typically the material is also subjected to mechanicalrubbing with e.g. one or more rotating brushes or specifiedroller(s)—after a while or after being treated with the developer.During processing the developer becomes loaded with components of thecoating that have been removed during development and the amount ofmaterial in the developer increases as more plates are developed. Due tothe increasing amount of dissolved material in the developer, theactivity of the developer decreases resulting in a reduced ability ofremoving the non-printing areas of the lithographic image.

After development, the plate is typically rinsed with water to removeany remaining developer and then gummed, which is sometimes also calledfinished or desensitized. Gumming involves the application of aprotective coating on the lithographic image, especially thenon-printing areas, to avoid contamination or oxidation of the aluminumsubstrate. Gum solution can be applied by immersion, by spraying or byjetting as disclosed for example in EP 1 524 113.

EP 1 696 274 discloses a method for automatic development of aphotosensitive lithographic printing plate precursor using an alkalidevelopment processing solution followed by a treatment with a gumsolution.

WO2007/057347 discloses a method of making a lithographic printing platewherein the precursor is washed in a prewashing station comprising twoor more prewashing units which have the configuration of a cascadesystem, whereby the wash liquid used for washing the precursor in thefirst and second prewashing unit are respectively present in a first anda second tank, and whereby the wash liquid of the second tank overflowsto the first tank when fresh water is added in the second prewashingunit.

An important trend in lithographic platemaking is related to ecology andsustainability. Systems and methods which enable a low consumption ofprocessing liquids such as developer, rinse water and/or gum solution,or which allow processing with aqueous developers comprising nohazardous chemicals and/or which have a pH close to 7 (neutraldeveloper), have attracted a lot of attention in the marketplace. Aconvenient method which has become popular involves the use of a gumsolution as developer, whereby the plate is developed and gummed in asingle step. Such methods however can only be used for speciallydesigned plates, which have lithographic coatings that are sufficientlysoluble or dispersible in the gum solution so that a good clean-out(complete removal of the coating at non-printing areas of the image) isobtained.

High alkaline developers may be aggressive towards the aluminiumsupport, more specific to the hydrophilic surface of a grained andanodized aluminium support, and are prone to form so-called “oxidation”or “scumming” spots. Theses spots, also referred to in the art as“artefact”, may accept ink at the non-image areas (i.e. toning), mayreduce the lithographic properties of the plate and/or may result in aslower restart. The formation of such oxidation spots is even morepronounced when silicate-free developers are used, and/or when duringprinting oxidative types of inks are used and/or when the level ofchlorine ions in the fountain/rinsing water is high.

SUMMARY OF THE INVENTION

Preferred embodiments of the present invention provide a method forprocessing lithographic printing plate materials with an alkalinedeveloper, which enable to reduce the consumption of processing liquidssuch as developer and/or gum solution and/or to reduce the amount ofwaste liquids generated during processing. Other preferred embodimentsof the present invention provide a method for making a lithographicprinting plate precursor whereby the formation of oxidation spots atnon-image areas is reduced or even prevented.

These advantages and benefits are realised by the method defined below,with preferred embodiments also defined below. The invention has thespecific feature that the printing plate material is developed with analkaline development solution, followed by two treatments with gumsolution, performed in a first gumming unit comprising a first gumsolution and subsequently in a second gumming unit comprising a secondgum solution, wherein both gumming units are configured as a cascade,i.e. a configuration wherein the second gum solution overflows into thefirst gum solution. In addition, it was found that the occurrence ofoxidation spots at the non-image areas of the surface of a grained andanodized aluminum support of the plate material, after exposure anddevelopment in an alkaline solution, is substantially reduced and/oreven prevented when at least one nitrate salt is present in the rinseand/or gum solution(s).

The method of the present invention requires no rinsing step between thedevelopment and the gumming of the plate, because the liquid which ispresent in the first gumming unit acts as a rinsing and/or neutralisingliquid. The alkaline developer and the plate coating ingredients whichare present therein due to the prior development of a number of plates,may be dragged along with the processed plate into the first gumsolution but this contamination of the gum solution does not affect thequality of the processed plate because the plate is subsequently treatedwith fresh gum solution in the second gumming unit.

In summary, the method of the present invention provides the majoradvantage of consuming low amounts of processing liquids and thusgenerating only small amounts of waste liquids. Therefore, the currentinvention is not only convenient and cost-efficient, but is alsofavorable from an environmental point of view.

The method can be used for processing any type of printing plate,negative- as well as positive-working. Positive-working printing platesare preferred.

Further advantages and benefits of the invention will become apparentfrom the description hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a preferred embodiment of anapparatus, shown as it is filled with developer and gum solution.

FIG. 2a is a more detailed representation of the development cavityviewed along the processing direction.

FIG. 2b is a more detailed representation of the development cavityviewed along the direction which is perpendicular to the processingdirection.

FIGS. 3a and 3b are schematic representations of protruding elements(ribs) provided on the bottom plate of the development cavity.

FIG. 4 is a schematic cross-section of suitable shapes of protrudingelements.

The numbers in the Figures refer to the following features of apreferred apparatus:

-   -   1 development section    -   2 gumming section    -   3 first gumming unit    -   4 second gumming unit    -   5 development unit    -   6 development cavity    -   7 cover plate    -   8 entry aperture    -   9 exit aperture    -   10 bottom plate including a first part (10A), a second part        (10B) and a bend (10C)    -   11 roller pairs: 11A and 11B (development section); 11C, 11D,        11E and 11F (gumming section); and 11G (drying section).    -   12 development solution    -   13 scavenger rollers    -   14 brush    -   15 spray bars 15B, 15C and 15D    -   16 first gum sump 16A and second gum sump 16B    -   17 cascade overflow    -   18 drain    -   19 drying section    -   20 protruding element    -   21 sidewall    -   22 sidewall    -   23 processing direction    -   24 drying means    -   25 first gum solution    -   26 second gum solution

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Definitions

Development section: part of an apparatus which comprises a developmentunit and preferably also a developer recirculation system and adeveloper regeneration system.

Gumming section: part of an apparatus which comprises a gumming unit,and preferably also a gum recirculation system and a gum regenerationsystem.

Development unit: vessel designed to hold development solutionoptionally including nip rollers and the like.

Fresh (development or gumming) solution: solution which has not yet beenused for processing a plate material.

Gumming unit: vessel designed to hold gum solution optionally includingnip rollers, scavenger rollers, brushe(s) and/or means for supplying gumsolution to the plate.

(Re)circulation system: system comprising the necessary pipes andpump(s) to generate a flow of developer or gum solution.

Regeneration system: system comprising the necessary pipes and pump(s)to supply regenerator liquid to a development unit or a gumming unit.

Replenishment solution: regenerator liquid used to control the activitylevel of the development solution or the gum solution.

(Re)start: the process of draining developer and/or gum solution fromthe development unit or gum unit respectively, followed by refilling thedevelopment unit and/or gum unit with fresh developer or fresh gumsolution respectively (the latter step taken alone is referred to as“start”).

Unless otherwise indicated, parameter values of a solution, e.g. pH,density, viscosity, conductivity, etc. are always measured at 25° C.

Development

According to the current invention, an (exposed) printing plate materialis developed by means of a suitable alkaline developer, also referred toas herein as “development solution”. In the development step, thenon-printing areas of the coating of the plate material are at leastpartially removed without substantially removing the printing areas. Inthe event the non-printing areas are not completely removed by thedevelopment, complete removal may be achieved by the treatment with thefirst and/or the second gum solution.

In order to reduce the contamination of the development section and therinsing section, alkaline developers which are silicate-free orsubstantially silicate-free are preferred. Indeed, silicate containingdevelopers may cause a high contamination of the gumming unit(s) bysludge formation such as salted-out compounds, precipitated orflocculated ingredients, gelation/turbidity (i.e. formation ofgelatinous and/or turbid areas) and/or other undissolved compounds.Sludge may cause problems such as clogging of pumps, deposit on the exitand/or other rollers, build-up on heater elements and/or intensivecleaning/maintenance of the gumming vessels. However, silicate-free orsubstantially silicate-free developers are known to be more aggressivetowards the hydrophilic surface of the support and are even more proneto form so-called “oxidation” or “scumming” spots. The occurrence ofthese spots, as discussed above, is reduced when the plate is treatedwith a gum solution including at least one nitrate salt.

Development of a plate material is typically performed in a vesselcontaining development solution, for example by dipping or immersing theplate in the developer, or by (spin-)coating, spraying and/or pouringdeveloper onto the plate. The treatment with development solution may becombined with mechanical rubbing, e.g. by one, two or more rotatingbrushes and/or specified rollers e.g. Molton rollers. As most preferredembodiment, the development is carried out by the apparatus describedhereafter. Preferably, the plate is not brushed during the treatmentwith alkaline development solution. During the development step, anywater-soluble protective layer on top of the image-recording layer, ifpresent, is preferably also removed.

During processing, the development solution becomes loaded withcomponents of the coating that have been removed by the development andthe amount of material in the development solution increases as moreplates are developed. Due to this increasing amount of material in thedevelopment solution, the activity of the development solution typicallydecreases which may result in a reduced ability to remove thenon-printing areas of the lithographic image and/or a reduced ability tomaintain the removed components in solution or in a dispersed state. Inaddition, the pH of the development solution may decrease due to thedissolution of carbon dioxide from the air into the development solutionas the time passes. Therefore, the development solution is preferablyshielded from the air by a cover plate.

In a preferred embodiment, a low amount (as defined below) ofdevelopment solution is used during a period of about one week or more,more preferably about two weeks or more, during which a plurality ofplates is processed with the same development solution, either with orwithout regeneration. After that period, the development unit isreloaded with fresh development solution. This process is preferablyfully automatic, which means that the development solution is drainedfrom the development unit and that the development unit is refilled withfresh developer by means of a system including a supply tank includingfresh development solution, a waste tank for collecting the exhausteddeveloper and the necessary pipes and pumps. The fresh developmentsolution may be produced automatically inside the processing apparatusby diluting a more concentrated solution with water.

Because the development solution is used during just a limited period oftime, formation of sludge—such as salted-out compounds, precipitated orflocculated ingredients and/or other undissolved compounds—during theprocessing period between two (re)starts may be further limited. Also,the level of dissolved ingredients and/or compounds present in thedeveloping solution may be limited; i.e. the development solution is notexhausted. As a result, not only the maintenance of the development unit(as described below) becomes less burdensome, but also deposit on theexit and/or other rollers, and/or build-up on heater elements in thedeveloper unit is limited as well as possible adherance of sludge on theprinting plate which may impair the images formed thereon; e.g. acceptink in the non-image areas.

The apparatus described below is especially suited to enable to use arelatively small volume of development solution during a limited periodof time between two (re)starts. In the context of this invention, a lowamount of development solution refers to for example a volume below 50 le.g. between 1 and 20 l, preferably between 2 and 15 l, more preferablybetween 5 and 12 l and most preferably between 8 and 10 l. The volumerefers to the amount of development solution present in the developmentunit, i.e. excluding the volume that may be present in the regenerationsystem, in the recirculation system and in any supply and wastecollector tanks. Said volume is dependent on the width of thedevelopment unit (which is typically between 0.5 m and 2.0 m), asexplained below.

Preferably, the development solution is reloaded after one week ofprocessing and/or after processing for example 400 m² of precursor.Preferably, the reloading of the development solution is automated.

Alternatively, the development quality may be kept constant for a longerperiod, so that a restart can be postponed for a longer time, forexample more than one month, preferably more than two months, morepreferably more than four months and most preferably more than sixmonths. In this embodiment, a low volume of development solution as wellas high volume of development solution may be used; however, a highvolume of development solution is preferred, for example a volumebetween 20 and 200 l, preferably between 40 and 150 l, more preferablybetween 50 and 100 l and most preferably between 60 and 90 l. As above,the actual amount depends on the width of the development unit.

The volume of the development solution in the development unit ispreferably in the range Vmin to Vmax, which both depend on the width ofthe development unit according to the following formulae:

Vmax=[B+(W/0.95 m)]·liter  (formula 1)

Vmin=[1+(W/0.95 m)]·liter  (formula 2)

wherein B represents an integer from 6 to 17 and wherein W is the width,expressed in meter and measured perpendicularly to the processingdirection of the largest plate material that can be processed in thedevelopment unit (wherein the “processing direction” is defined as thepath in the development unit along which the plate material travelsduring the treatment with development solution). Preferably B represents6, 7, 8, 9 to 13, 14, 15, 16 or 17.

Regeneration of Development Solution

The activity level of the development solution may be maintained duringits working period by adding replenishment solution. Depending on theconcentration of the mentioned regenerator liquids, the rate ofregeneration may be between 1 ml and 100 ml per m² of treated platematerial, preferably between 2 ml/m² and 85 ml/m², 4 ml/m² and 60 ml/m²,more preferably between 5 ml/m² and 30 ml/m².

It has been found that by using small amounts of developer for a limitedperiod in time, little replenishment is required to keep the activity ofthe developer at a sufficient level and/or constant. Therefore, theembodiment wherein a small volume of developer is used generates,compared to development of the prior art where large amounts ofdeveloper for a longer period in time are used, less waste. Indeed, thewaste—including the amount of drained developer and the amount ofapplied replenisher—generated during said limited period in time, isless compared to the waste that would have been generated when thedevelopment would have been carried out during a longer period in time.

In addition, the volume of development solution is preferably keptconstant by for example adding water and/or development solution; alsoreferred to in the art as top-up the development solution.

The mentioned regenerator liquids can be added continuously, after apredetermined period of time or in batches when the activity of thedevelopment solution becomes too low and/or to keep the activity levelconstant. The activity level of the development solution can bedetermined by monitoring e.g. pH, density, viscosity, conductivity, thenumber and/or area (square meters) of processed plates processed since a(re)start with fresh solution and/or the time lapsed since a (re)startwith fresh solution. When the addition of regenerator is regulated bymeasurement of one of these parameters, for example the conductivity ofthe development solution, the regenerator liquid can be added when apredetermined threshold value of that parameter is reached or iscrossed. The amount of regenerator added each time depends on thepredetermined threshold value. For example, when the measured parameteris the number of square meters of plate material processed, apredetermined amount of replenishment is added each time afterprocessing a predetermined area of plate material. As a further example,the measured parameter can be the conductivity or conductivity increaseof the solution monitored with a conductivity meter. Beyond a definedconductivity value, regenerator can automatically be added to thedevelopment solution.

The development unit preferably contains an overflow pipe which drainsthe development solution into a collector tank. The drained developmentsolution may be purified and/or regenerated by e.g. filtration,decantation or centrifugation and then reused, however, the draineddevelopment solution is preferably collected for disposal.

Recirculation of Development Solution

The development solution present in the development unit can becirculated, e.g. by means of a circulation pump. In it most simple form,circulation means that a flow of development solution is generatedwithin the development unit, preferably producing sufficient turbulenceto enhance the removal of non-printing areas from the coating of theplate. As a result, during the treatment with the development solution,application of one or more brush(es) during the processing step is notrequired. The development solution may be sucked in via an outlet of thedevelopment unit, preferably near the exit rollers of the developmentunit, from where it may be drained to a waste collector tank.

According to a more preferred embodiment, at least a part of thedevelopment solution is not drained but recirculated, i.e. conveyedalong a closed loop, e.g. from a sump of the development unit into oneor more inlet openings such as for example spray or jet nozzles (asdescribed further below), which apply the developer onto the plateand/or onto an optional brush which is in contact with the plate. Excessof developer then flows from the plate back into the sump. The mostpreferred embodiment of such recirculation involves pumping thedeveloper into a development cavity, as described below.

During recirculation, the development solution is preferably at leastpartly removed (sucked) from the development unit and then injectedthrough at least one inlet opening formed in for example the sidewall ofthe development cavity back into the development unit (or cavity, seefurther), thereby circulating and stirring the development solution.More preferably, the development solution which is sucked away isinjected through at least one inlet opening in the development unit nearthe exit roller pair. Even more preferably, the development solutionwhich is sucked away is injected through at least one inlet openingformed in the cover plate of the development unit and/or cavity. Mostpreferably, the development solution which is sucked away is injectedthrough at least one spray bar which is preferably positioned in thedevelopment unit near the exit roller pair, more preferably parallel tothe exit rollers. The development solution is preferably at least partlysucked in from the area under and/or near the exit rollers in thedevelopment unit. Preferably, a filter is present in the circulationsystem, e.g. in the pipes, which is capable of removing sludge and/ordissolved ingredients from the development solution.

Development Solution

Any type of alkaline developer may be used in the method of the presentinvention, depending on the type of printing plate that is processed.Solvent-based or aqueous alkaline developers may be used. Solvent baseddevelopers have mainly been used to develop negative-working platematerials, while positive-working plate materials typically require ahighly alkaline developer without much solvent therein.

Unless otherwise indicated, the amounts of developer ingredients givenherein refer to the fresh developer as used for a (re)start. Such freshdeveloper may be obtained as a ready-to-use solution or by diluting amore concentrated solution that is supplied by the manufacturer withwater, e.g. a dilution between 2 and 10 times. The dilution of adeveloper concentrate may be done in a separate apparatus or may beintegrated in the processing apparatus. As a result, the preferredembodiments of this invention allow to develop plates with goodclean-out by using less than 150 ml/m² of such concentrated solution,preferably less than 50 ml/m², more preferably less than 25 ml/m² andmost preferably from 0.5 to 10 ml/m² of such concentrated solution.Alternatively, 0.2 to 2 ml/m² of developer is preferably used.

A preferred alkaline developer is an aqueous solution which has a pH ofat least 10, more typically at least 12, preferably from 13 to 14.Preferred high pH developers comprise at least one alkali metalsilicate, such as lithium silicate, sodium silicate, and/or potassiumsilicate. Sodium silicate and potassium silicate are preferred, andsodium silicate is most preferred. A mixture of alkali metal silicatesmay be used if desired. Especially preferred high pH developers comprisean alkali metal silicate having a SiO₂ to M₂O weight ratio of at leastof at least 0.3, in which M is the alkali metal. Preferably, the ratiois from 0.3 to 1.2. More preferably, it is from 0.6 to 1.1, and mostpreferably, it is from 0.7 to 1.0. The amount of alkali metal silicatein the high pH developer is typically at least 20 g of SiO₂ per 1000 gof developer (that is, at least 2 wt. %) and preferably from 20 g to 80g of SiO₂ per 1000 g of developer (2-8 wt. %). More preferably, it is 40g to 65 g of SiO₂ per 1000 g of developer (4-6.5 wt. %).

In a highly preferred embodiment, as an alternative for the alkali metalsilicate, alkalinity is provided by a suitable concentration of anysuitable base. Such developers are referred to as “silicate-free”developers. Suitable bases include ammonium hydroxide, sodium hydroxide,lithium hydroxide, potassium hydroxide and/or organic amines, and/ormixtures thereof. A preferred base is sodium hydroxide. Suchsilicate-free developers do substantially exclude silicates; they aresubstantially silicate-free developers. The word “substantially” meansthat the presence of unavoidable impurities, minute silicates asbyproduct and/or very small amounts which might have been added to thedevelopment solution, are tolerated. Very small amounts refer to forexample less than 1% wt, preferably less than 0.5% wt and mostpreferably less than 0.1% wt, based on the total weight of thedevelopment solution.

Solvent-based alkaline, silicate-free, developers preferably have a pHabove 9, more preferably above 9.5, and most preferably above 10.Solvent-based developers comprise water and an organic solvent or amixture of organic solvents. They are substantially free of silicates,(alkali metal) hydroxides, and mixtures of silicates and (alkali metal)hydroxides. The developer is preferably a single phase. Consequently,the organic solvent or mixture of organic solvents is preferably eithermiscible with water or sufficiently soluble in the developer so thatphase separation does not occur.

The following organic solvents and mixtures thereof are suitable for usein solvent-based developers: the reaction products of phenol withethylene oxide (phenol ethoxylates) and with propylene oxide (phenolpropoxylates), such as ethylene glycol phenyl ether (phenoxyethanol);benzyl alcohol; esters of ethylene glycol and of propylene glycol withacids having six or fewer carbon atoms, and ethers of ethylene glycol,diethylene glycol, and propylene glycol with alkyl groups having six orfewer carbon atoms, such as 2-ethoxyethanol, 2-(2-ethoxy)ethoxyethanol,and 2-butoxyethanol. A developer that comprises phenoxyethanol ispreferred. The developer typically comprises 0.5 wt % to 15 wt %,preferably 3 wt % to 5 wt % of the organic solvent or solvents, based onthe weight of the developer.

A suitable alternative developer for processing positive-working platescomprises a non-reducing sugar and a base. Such alkaline developerspreferably have a pH above 9, more preferably above 10, and mostpreferably above 12. The term “non-reducing sugar” means a saccharidewhich is free of free aldehyde or ketone groups and thus is notreducing, e.g. trehalose type oligosaccharides, glycosides and sugaralcohols obtained by hydrogenating and reducing saccharides. Examples ofthe trehalose type oligosaccharides include saccharose, and trehalose.Examples of the glycosides include alkyl glycoside, phenol glycoside,and mustard oil glycoside. Examples of the sugar alcohols include D,L-arabitol, ribitol, xylitol, D, L-sorbitol, D,L-mannitol, D,L-iditol,D,L-talitol, dulcitol, and arodulicitol. Further, maltitol obtained bythe hydrogenation of disaccharide or reduced material (reduced starchsyrup) obtained by the hydrogenation of oligosaccharide may be used.Preferred among these non-reducing sugars are sugar alcohols andsaccharose. Even more desirable among these non-reducing sugars areD-sorbitol, saccharose, and reduced starch syrup because they havebuffer action within a proper pH range.

These non-reducing sugars may be used alone or in combination of two ormore thereof. The proportion of these non-reducing sugars in thedeveloper is preferably from 0.1 to 30% by weight, more preferably from1 to 25% by weight.

The aforementioned non-reducing sugar may be used in combination with analkaline agent as a base, properly selected from the group consisting ofknown materials such as inorganic alkaline agents, e.g. sodiumhydroxide, potassium hydroxide, lithium hydroxide, trisodium phosphate,tripotassium phosphate, triammonium phosphate, disodium phosphate,dipotassium phosphate, diammonium phosphate, sodium carbonate, potassiumcarbonate, ammonium carbonate, sodium hydrogencarbonate, potassiumhydrogencarbonate, ammonium hydrogencarbonate, sodium borate, potassiumborate and ammonium borate, potassium citrate, tripotassium citrate, andsodium citrate.

Further preferred examples of alkaline agents include organic alkalineagents such as monomethylamine, dimethylamine, trimethylamine,monoethylamine, diethylamine, triethylamine, monoisopropylamine,diisopropylamine, triisopropylamine, n-butylamine, monoethanolamine,diethanolamine, triethanolamine, monoisopropanolamine,diisopropanolamine, ethyleneimine, ethylenediamine and pyridine.

These alkaline agents may be used singly or in combination of two ormore thereof. Preferred among these alkaline agents are sodiumhydroxide, potassium hydroxide, trisodium phosphate, tripotassiumphosphate, sodium carbonate and potassium carbonate.

Another alternative silicate-free and sugar-free alkaline aqueousdeveloper composition has a pH of at least 12 and comprises (a) ahydroxide, (b) a metal cation M2′ selected from barium, calcium,strontium, and zinc cations, (c) a chelating agent for the metal cationM⁺ and (d) an alkali metal salt different than all of a, b, and c above.

Optional components of all the above mentioned developers are e.g.anionic, nonionic and/or amphoteric surfactants, biocides (antimicrobialand/or antifungal agents), antifoaming agents or chelating agents (suchas alkali gluconates), solubilizers, image protecting agents such asblockers or retardants, dissolution inhibitors and thickening agents(water soluble or water dispersible polyhydroxy compounds such asglycerin or polyethylene glycol).

Gumming

According to the present invention, the development described above isfollowed by at least two treatments with a gum solution, which isapplied by means of a cascading gumming section comprising a first and asecond gumming unit wherein a first and second gumming step are carriedout respectively. This gumming section is also referred to as the“gumming system”.

In the first gumming step, the processed plate is treated with a firstgum solution. The main purpose of this treatment is to rinse and/orneutralise the plate, i.e. the removal of any developer from the surfaceof the plate, and to ensure good clean-out of the image, if not alreadyobtained in the development unit. In the second gumming step, the platematerial is subsequently treated with a second gum solution. The mainpurpose of the second step is to protect the lithographic image by theapplication of a gum layer as further discussed below. It shall beunderstood, however, that the said purpose of the first and secondgumming steps is not a limitation of the present invention. Forinstance, also the second gum solution may contribute to the clean-outof the image, for those plate materials of which the non-printing areasof the coating are not completely removed after the first gumming step.Reduced clean-out usually results in toning (ink-acceptance in thenon-image areas) of the printing plate and/or in ink build-up on theblanket.

The gum solutions are preferably brought into contact with the printingplate by spraying, jetting, immersing, dipping or by a coatingtechnique, including spin coating, roll coating, slot coating or gravurecoating. The use of spray bars is preferred. A spray bar typicallyincludes a hollow rod with a predetermined series of holes. The gummingunit(s) may also be provided with at least one roller for rubbing and/orbrushing the plate while applying the gum to the coating.

The two gumming steps are carried out in two different gumming unitsconfigured as a cascade whereby the second gum solution overflows intothe first gumming unit. Such a cascade configuration provides theadvantage that sludge formation and/or contamination by for examplecarry-over of dissolved ingredients in the second gum solution isreduced, whereby an increase of the viscosity of the gum solution in thesecond gumming unit can be reduced or inhibited. This results in animproved lifetime of the gumming system as only the gum solution of thefirst gumming unit becomes loaded with contaminants from the dragged-outdevelopment solution, whereby the second gum solution can be used forgumming a higher number of plates so as to save costs and to enable asustainable system.

During the use of the method of the present invention, the compositionsof the two gum solutions may be different, although the first gumsolution originates from the second gum solution via the cascadeoverflow. The difference may be due to for example contamination bydeveloper dragged out with the plate from the development unit into thefirst gumming unit and/or by further dissolution of non-printing areasof the coating if clean out is not fully achieved by the development,further combined with for example insufficient regeneration of the firstgum solution by the cascade overflow. The latter problem may be solvedby actively pumping gum solution—in addition to the cascadeoverflow—from the second to the first gumming unit.

(Re)Circulation of Gum Solution

The first and/or second gum solutions are preferably (re)circulated,more preferably independently from one another. The first and second gumsolutions are kept in respectively two baths or sumps from which theyare recirculated into for example spray bars which supply the gumsolution. The gum solutions then flow back into the respective sumps.

Preferably, a filter is present in the (re)circulation system, e.g. inthe pipes, which is capable of removing any kind of sludge and/ordissolved ingredients from the gum solutions. Regeneration of gumsolution

The gum solutions may be regenerated by adding water, a replenishmentsolution or optionally diluted fresh gum solution, or a mixture thereof.Adding optionally diluted fresh gum solution is preferred.

A concentrated replenishment solution can be added as replenishmentsolution when the concentration of active products is under a desiredlevel in the gum solution. A diluted replenishment solution or water canbe used when the concentration of active products is above a desiredlevel in the gum solution and/or when the viscosity of the gum solutionis increased or when the volume of the gum solution is under a desiredlevel, e.g. due to evaporation of the solvent or water.

The above mentioned regenerator liquids may be added to the first and/orsecond gum solution. The amount of regenerator added to the first gumsolution may be restricted so as to compensate only for the volume whichis drained in the cascade and dragged out with the plates. The amount ofregenerator added to the second gum solution is preferably adjusted tocompensate for the degradation of the gum solution by the dragged-outdeveloper and for the volume which is drained as waste.

It is preferred that the amount of replenishment and/or gum solutionadded for the regeneration of gum solution, is small in order to limitthe amount of waste produced during processing. Therefore, the rate ofregeneration—depending on the concentration of the replenishment/gumsolution—is preferably between 1 ml and 100 ml per m² of treated plates,more preferably between 2 ml/m² and 85 ml/m², more preferably between 4ml/m² and 60 ml/m² and most preferably between 5 ml/m² and 30 ml/m².

The addition of regenerator, i.e. the type and the amount thereof, maybe regulated by the measurement of for example the number and/or area ofprocessed plates, the pH or pH change of the gum solution, theviscosity, the density, the time lapsed since the gumming system wasloaded with fresh gum solution, or by monitoring the minimum and maximumvolume in each gumming unit, or a combination of at least two of them.

The first gumming unit preferably contains an overflow pipe which drainsthe gum solution into a collector tank. The drained gum solution may becleaned by e.g. filtration, decantation or centrifugation and thenreused to regenerate the first and/or the second gum solution.Preferably however, the drained first gum solution is collected fordisposal.

Gum Solution

The composition of the gum solution described hereafter refers to thefresh gum solution that is used for a (re)start. Preferably, the samegum solution is used for the (re)start in both units of the gummingsection. In alternative embodiments, a (re)start may involve filling thefirst and second gumming unit with different gum solutions. In thatcase, the composition of the gum solution described herein refers to thefresh gum solution used in the second gumming unit. Such fresh gumsolution may be obtained as a ready-to-use solution or by diluting amore concentrated solution that is supplied by the manufacturer. Thedilution of a gum concentrate may be done in a separate apparatus or maybe integrated in the processing apparatus.

Preferably, the second gum solution is reloaded after one week ofprocessing and/or after processing for example at least 400 m² ofprecursor. Preferably, the reloading of the first and/or second gumsolutions are automated.

Alternatively, the gum quality may be kept constant for a longer period,so that a restart can be postponed for a longer time, for example morethan one month, preferably more than two months, more preferably morethan four months and most preferably more than six months.

Suitable gum solutions, to be used as fresh gum solution in the presentinvention, are aqueous liquids which comprise one or more surfaceprotective compounds that are capable of protecting the lithographicimage of a printing plate against contamination, oxidation or damaging.The aqueous solution further contains at least one nitrate salt such asfor example Li, Na, K, Rb, Ce, Be, Sr, Mg, Ba, Ti, Zr, Sc, Vo, Cr, Mn,Fe, Co, Ni, Pd, Cu, Ag, Zn, Cd, Hg, B, Al, Ga, Tl, and/or Pb nitrate.Any nitrate salt is suitable, sodium or potassium nitrates arepreferred.

Suitable examples of surface protective compounds are film-forminghydrophilic polymers or surfactants. The layer that preferably remainson the plate after treatment with the gum solution in the second gummingstep and drying preferably comprises between 0.1 and 20 g/m² of thesurface protective compound. This layer preferably remains on the plateuntil the plate is mounted on the press and is removed by the ink and/orfountain when the press run has been started. The gum solutionspreferably have a pH below 11, more preferably below 9, even morepreferably a pH from 0 to 8, and most preferably from 1 to 6. Suitablegum solutions used herein have a pH around 2, 5 or 7.

A solution of a non-ionic surfactant and/or a solution of a buffer canbe added when the gum solution needs a higher concentration of asurfactant or when the pH of the gum solution needs to be controlled ata desired pH value or at a desired pH value in a range of two pH values,e.g. between 1 and 9.

Processing Apparatus

An apparatus which is especially designed for performing the processingmethods of the present invention is described hereafter.

The Figures represent a highly preferred embodiment of such a processingapparatus, which includes a development section (1) and a gummingsection (2) comprising two gumming units (3,4) mutually connected by acascade (17) which allows overflow of liquid from the second gummingunit into the first gumming unit. The development section (1) preferablyincludes a development unit (5) comprising an essentially closeddevelopment cavity (6) comprising a cover plate (7), a bottom plate (10)and sidewalls (21,22).

Well known features which are preferably present in the developmentsection of the apparatus but not shown in the Figures are: a feeder fordelivering plates one by one to the development section; a(re)circulation and/or regeneration system; supply tanks comprisingfresh developer, fresh gum solution, or one or more replenishingsolutions; waste collector tanks wherein exhausted developer or gumsolution are drained; a water tank to dilute concentrated chemistry; andother conventional parts.

When the description below refers to the plate material which during theoperation of the apparatus travels through the various sections, it isassumed that the plate is facing upwards, i.e. with the heat- orlight-sensitive coating facing upwards (the other side of the plate isreferred to as “backside”). However, embodiments wherein the plate isfacing downwards are equally within the scope of the present invention.

Preferred Processing Apparatus: Development Section

The development section (1) includes a development unit (5) whichpreferably comprises at least two roller pairs (11A, 11B)—also referredto as nip or feeder rollers—which convey the plates into and out of thedevelopment unit. The development unit preferably comprises a coverplate (7) to shield the development solution from the air.

Preferably, an entry roller pair (11A) feeds the plate into thedevelopment unit, more preferably into a development cavity (6) of theunit, which is an essentially closed volume defined by a bottom plate(10), a cover plate (7) and sidewalls (21,22). The cavity has an entryaperture (8) where the plate enters the cavity and an exit aperture (9)where the plate leaves the cavity. An exit roller pair (11B) preferablyconveys the plate from the development section to the gumming section.

A rubber blade may be provided at the entry aperture to prevent air fromflowing into the cavity. The development cavity is preferably completelyfilled with development solution without any air being present betweenthe cover plate and the surface of the development solution. Preferably,the cover plate covering the development cavity is completely in contactwith the liquid surface of the development solution so that any flow ofair above the development solution—i.e. the flow of air from the entryaperture to the exit aperture—is cut off. The main function of the coverplate is to reduce possible degradation of the development solution bythe absorption of carbon dioxide from the ambient air and/or evaporationof water, thereby allowing to reduce the rate of regeneration (if any).The cover plate may also extend beyond the entry or exit aperture, e.g.the cover plate may include arc-shaped curves or rectangular shapeswhich cover the upper peripheral surfaces of the nip rollers.

The volume of the development cavity is preferably as low as possible.Preferably the volume of the cavity is from 0.5 dm³ to 50 dm³; morepreferably from 1 dm³ to 25 dm³ and most preferably from 2 to 10 dm³. Ina preferred embodiment, the entry aperture (8) and exit aperture (9) arenarrow slots which have an aspect ratio (height/width) of at least 10,more preferably at least 20. The height of the entry slot (8) ispreferably between 2 and 5 times the thickness of the plate. The exitslot (9) is preferably more narrow, for example having a height only afew times (for example 2 to 3) bigger than the thickness of the plate.

The bottom plate (10) preferably includes at least two parts which areseparated by an upward bend (10C) so that a first part of the bottomplate (10A) is oriented at an angle from 0.5° to 60° relative to asecond part of the bottom plate (10B). More preferably, the angle isbetween 1° and 50°, more preferably between 5° and 45° and mostpreferably between 10° and 35° relative to the first part. The length(distance along the processing direction) of the first and/or secondpart of bottom plate is preferably adapted in order to obtain a smoothmovement of the plate through the development cavity. Preferably, thefirst part (10A) has a length from 0 to 50 cm, more preferably from 1 to30 cm and most preferred from 2 to 15 cm. The second part (10B)preferably has a length from 1 to 50 cm, more preferably from 2 to 30 cmand most preferably from 3 to 25 cm. Preferably the upward bend issubstantially perpendicular relative to the processing direction.

The surface of the bottom plate (10), which faces the inside of thedevelopment cavity, is preferably provided with one or more protrudingelements (20), which maintain a distance between the backside of theplate and the bottom plate. Preferably, at least two protruding elementsare present, more preferably at least three protruding elements arepresent and most preferably at least four protruding elements arepresent. As a result, formation of scratches at the backside of theplates is reduced and a smooth transport of the plate through the cavityis obtained. In addition, the protruding surface of the bottom plate mayprevent contact between the plate and sludge such as salted-outcompounds, precipitated or flocculated ingredients which are collectedbetween the protruding elements.

The protruding elements may have any shape, e.g. spherical, rectangular,oval, triangular or longitudinal. FIG. 4 illustrates suitable shapes ofprotruding elements. Preferred protruding elements are elongated ribs.

Preferably the cover plate is provided with at least two elongated ribs;more preferably at least three and most preferably at least four. Theseelements may be positioned parallel to each other. The length of theelongated rib(s) is preferably between 1 mm and 25 cm, more preferablybetween 5 mm and 15 cm and most preferably between 10 mm and 10 cm. Thelength may be at least the sum of the length of 10A and 10B. The heightof the elongated rib(s) is preferably at least 0.1 mm and at most 50 mm,more preferably between 0.1 mm and 10 mm and most preferably between 1mm and 5 mm. The elongated rib(s) may be oriented at an angle relativeto the processing direction. Such elongated ribs may be parallel to theprocessing direction of the plate, indicated by the arrow (23) in FIG.3, but are more preferably oriented at an angle relative to theprocessing direction. Said angle (α in FIG. 3b ) is for example 1 to 45°preferably 5 to 35° and most preferably 10 to 25° relative to theprocessing direction. Alternatively, the angle α may have a differentvalue for one or more ribs, or in other words the ribs may be not fullyparallel relative to each other.

In the preferred embodiment of FIG. 2a , the protruding elements (20)have a trapezoidal cross-section with a rounded top. The height of theprotruding elements—measured at the highest part in case of spherical,round or oval shapes—is preferably at least 0.1 mm and at most 50 mm,more preferably between 1 mm and 10 mm and most preferably between 1 mmand 5 mm.

These elements may be positioned for example ad random, grouped in amatrix, or along parallel lines. Such lines may be parallel to theprocessing direction of the plate but are more preferably oriented at anangle relative to the processing direction. Said angle (α; asillustrated for elongated ribs in FIG. 3b ) is for example 1 to 45°preferably 5 to 35° and most preferably 10 to 25° relative to theprocessing direction. Alternatively, the angle α may have a differentvalue for one or more lines, or in other words the lines may be notfully parallel relative to each other. The length of the lines ispreferably between 1 mm and 25 cm, more preferably between 5 mm and 15cm and most preferably between 10 mm and 10 cm.

The protruding elements may be made from metal, fiber, and/or otherflexible/ductile materials. The relief may be extruded, oriented,expanded, woven or tubular and can be made from polypropylene,polyethylene, nylon, PVC or PTFE. A metal relief may be woven, knitted,welded, expanded, photo-chemically etched or electroformed from steel orother metals.

The development cavity preferably does not include one or more rollerpairs which are typically present in a development unit of the priorart. Due to the special design of the development cavity of the presentinvention as described above, the plate is transported through the unitparallel or nearly parallel with the bottom plate which is preferablyconfigured with an upward bend (see above) to guide or push up the platewithout the need for transport rollers. During transport, the plate maybe in contact with the bottom plate.

The development cavity preferably does not include one or more rollerpairs which are typically present in a development unit of the priorart. Due to the special design of the development cavity of the presentinvention as described above, the plate is transported through the unitparallel or nearly parallel with the bottom plate which is preferablyconfigured with an upward bend (see above) to guide or push up the platewithout the need for transport rollers. During transport, the plate maybe in contact with the bottom plate.

In a preferred embodiment, the cover plate is also provided with any ofthe above described protruding elements. In another embodiment, only thecover plate is provided with the above described protruding elements.The protruding elements on the cover plate may induce a turbulence inthe development solution whereby the formation of so-called boundarylayers and/or laminar flow are steered in order to achieve optimaldevelopment.

As described above, the development solution is preferably regeneratedby means of an inlet that supplies regenerator liquid to developmentunit (5) and/or development cavity (6). Other well known elements of theregenerator system are not shown in the Figures, such as a supply tankfor replenishment solution; a pump and the necessary pipes to supply theregenerator liquid to the development unit (5) and/or development cavity(6).

Preferred Processing Apparatus: Supply of Developer by Nozzles

In order to provide sufficient turbulence within the development unit,the developer is preferably applied onto the printing plate by means ofnozzles which spray or jet a flow of developer on the surface of theplate. The nozzles may be configured as an array of nozzles, e.g. anarray of holes in a spray bar or an array of jet nozzles in an inkjethead, e.g. a valve-jet head.

The use of nozzles is especially suitable for the embodiment wherein thedevelopment unit comprises a development cavity as described above. Inthat embodiment, the nozzles may be integrated in a sidewall or in bothsidewalls of the development cavity so as to discharge developmentsolution transversely over the coating of the plate. In the alternative,the nozzles may be present in the bottom or the cover plate, dependingwhich of both is facing the image recording layer of the printing plate.Combined embodiments wherein nozzles are integrated in one or bothsidewalls as well as in the bottom and/or the cover plate are alsowithin the scope of this invention.

The developer is preferably supplied by the nozzles as a pressurizedflow over the surface area of the plate such that successive targetareas of the plate are dynamically and uniformly flooded withdevelopment solution. The nozzle streams of development solution can betuned with respect to direction, shape, overlap, and surface turbulence.Although the plate target area preferably experiences a continuousturbulent flooding, the supply through the nozzles can also be appliedin consecutive pulses. Dissolution of the soluble coating regions isthereby achieved quickly and uniformly by providing a flow of developerliquid which causes turbulence and which is constantly displaced andreplaced.

At sufficient volumetric flow rate, the development solution isconstantly displaced at the surface of the plate during the developmentdwell time, whereby no boundary layer forms on and travels with theplate and each unit volume of coating is rapidly and uniformlyprocessed. Preferably, depending on the speed at which the plate travelsthrough the development unit, a turbulent flow of development solutionis applied for a short dwell time onto each unit area of the coatedplate; for example, at a speed between 0.5 and 5 m/min, a dwell time ofless than about 30 seconds, more preferably a dwell time between 5 and25 seconds and most preferably a dwell time between 8 and 15 seconds.These figures are only a practical guideline and may be outside theseranges.

The use of brushes is not required in order to obtain fast and efficientdevelopment of the plates. In a preferred embodiment, the developmentcavity does not contain any brushes whereby the risk of scratches on theimage areas and/or maintenance (cleaning) of the brushes are eliminated.

Suitable spray nozzles are commercially available in many sizes andconfigurations, e.g. from Spraying Systems Co. (Wheaton, Ill., USA).Important parameters of the spray nozzles are the flow rate, the spraypressure, the drop size, the spray pattern and the spray nozzlealignment. Useful spray pressures are in the range of 1 to 5 bar, morepreferably from 1.5 to 2.5 bar. A preferred spray pattern is atapered-edge flat pattern because it can provide a uniform coverage overthe entire plate area as a result of overlapping distributions. Theangle of the spray cone and the spray distance between the spray nozzleand the plate define the target area on the plate. The nozzles may havea spray angle from 5° to 170°, the larger angle producing a large targetarea for a given spray distance. The nozzle target area on the platedepends on the spray angle and the spray distance and may be up to 15cm, which can be achieved by a nozzle having e.g. a spray angle of 110°and a spray distance of 5 cm. However a smaller target area ispreferred, e.g. less than 5 cm which may be achieved by a nozzle N witha spray angle of 50° and a 5 cm spray distance or 30° and 10 cmrespectively. Suitable drop sizes of the spray are from less than 1 mm,e.g. 100 μm (achieved by so-called atomizing nozzles), up to a few mm,e.g. from 1 to 5 mm, preferably from 1 to 2 mm. The drop size is mainlydetermined by the spray pressure and of course the properties of thedeveloper liquid.

The spray nozzles are preferably made of a material which is resistantto the developer liquid and provides a long wear life, e.g. stainlesssteel, a ceramic or a carbide. More information about spray nozzles canbe found in e.g. the books “Industrial Sprays and Atomization”,Springer, 1st edition (Sep. 17, 2002) and “Handbook of Atomization andSprays”, Springer, 2011.

Especially when high-resolution nozzles, i.e. nozzles with a very smalltarget area on the plate such as the nozzles of an inkjet head, areused, more intelligence can be built into the apparatus by supplyingimage data from the platesetter or the workflow software to the digitalcontroller of the apparatus of the present invention. Image-controlleddevelopment can be achieved in the apparatus of the present invention bya digital controller wherein the average dot coverage at the target areaof each nozzle, which is a portion of the image, is calculated and whichadjusts the volume of developer deposited on that target area inaccordance with said average dot coverage. In such embodiment, nodeveloper is deposited on “full-black” portions of the image, i.e.portions which consist entirely of printing areas, and a sufficientamount of developer is deposited on the gray and white portions of theimage, wherein said amount is made proportional to the average dotcoverage of said gray and white portions. More details concerningsuitable nozzles can be found in EP 2 775 351 (for example [0034] to[0049]).

Preferred Processing Apparatus: Gumming Section

The gumming section of the processing apparatus contains at least twogumming units which are provided in a cascade configuration, which meansthat the gum solution overflows from the second gumming unit into thefirst gumming unit. Additional gumming units may be used, but thepreferred embodiments comprise only two gumming units. Preferably, thefirst gumming unit does not allow overflow to the development section.

Each gum solution is applied to the printing plate by a spraying,jetting, dipping or coating technique, including spin coating, rollcoating, slot coating or gravure coating. The use of spray or (valve)jet nozzles is preferred. All features of the nozzles described abovefor supplying development solution equally apply to preferredembodiments for depositing gum on the plate, possibly in accordance withthe plate area or even with the image data of the plate, as described inEP 2 775 351.

In the preferred embodiment of FIG. 1, the nip rollers (11C, 11D) of thefirst gumming unit are provided with a scavenger roller (13) to preventcontamination of gum into the developer unit. Two spray bars areprovided in the first gumming unit: one bar (15B) which is capable ofspraying gum both onto the nip of the roller pair (11C) and onto thebrush (14) which is configured to apply gum onto the image of the plate,and one bar (15C) which sprays gum towards the nip of the roller pair(11D). The bars spraying gum to the nip of the roller pair preferablycontain at least one row of holes; the bar (15B) capable of spraying gumboth onto the roller and brush (14) preferably contains at least tworows of holes. Preferably, the bar(s) for spraying the first gumsolution, more preferably bars (15B) and (15C) are in a so-calledjog-mode, i.e. gum is provided on a regular basis even when no plate ispresent in the gumming unit in order to prevent stickiness of the niprollers and/or brush. Preferably, the nip rollers are engaged on aregular basis; even when no plate passes. The second gumming unitfurther includes a spray bar (15D) which is capable of keeping both niprollers in the second unit (11E, 11F) wet and which provides a finishinglayer onto the surface of the plate. This spray bar may also be in thejog-mode.

As described above, the second gum solution is preferably regenerated bymeans of an inlet that supplies regenerator liquid, which may be water,optionally diluted fresh gum and/or replenishment solution, to thesecond gumming unit, e.g. to the sump (16B). Other well known elementsof the regenerator system are not shown in the Figures, such as a supplytank for holding fresh gum solution, water or replenishment solution; apump and the necessary pipes to supply the regenerator liquid to thesecond gumming unit. Also the first gum solution may be regenerated,either by the same or an analogous regeneration system as used for thesecond gum solution. The first gum solution may also be regenerated byactively pumping gum solution from the second to the first gumming unit.

Preferred Processing Apparatus: Drying Section

After the final gum has been applied, the plate is preferably not rinsedbut immediately conveyed to a drying section which is preferablyintegrated into the apparatus. Drying can be achieved by means (24) foremitting hot air, infrared and/or microwave radiation, and other methodsgenerally known in the art. The plate may then be mounted on the platecylinder of a printing press and the printing process may be started.

Lithographic Printing Plate Materials

Any type of heat- and/or light-sensitive plate materials can beprocessed according to the methods and with the apparatus of the presentinvention. Preferred materials are positive- or negative-working platematerials which require alkaline processing. Positive-workingheat-sensitive materials are highly preferred.

Support

The preferred support of the lithographic printing plate material usedin the present invention has a hydrophilic surface or is provided with ahydrophilic layer. A particularly preferred lithographic support is agrained and anodized aluminum support. Preferably the aluminum isgrained by electrochemical graining in a solution comprising for examplenitric acid and/or hydrochloric acid. The aluminum is preferablyanodized by means of anodizing techniques employing sulphuric acidand/or a sulphuric acid/phosphoric acid mixture whereby an aluminumoxide layer (Al₂O₃) is formed. By anodising the aluminium support, itsabrasion resistance and hydrophilic nature are improved. Themicrostructure as well as the thickness of the Al₂O₃ layer aredetermined by the anodising step, the anodic weight (g/m² Al₂O₃ formedon the aluminium surface) is comprised between 1 and 8 g/m².

In a highly preferred embodiment the aluminum oxide surface is silicatedby treating its surface with a sodium silicate solution at an elevatedtemperature. Indeed, it has been observed that silicate-free developersoften are more aggressive towards the aluminium oxide layer of thesupport and may reduce lithographic properties of the plate which can becounteracted by post treating the aluminium support with an aqueoussolution containing silicates.

Preferably, said aqueous solution includes a compound containing asilicate anion and one or more cations. The silicate anion is preferablyan anion in which one or more central silicon atoms are surrounded byelectronegative ligands such as for example fluor or oxygen atoms. Thesilicate anion is preferably selected from phosphosilicates,orthosilicates, metasilicates, hydrosilicates, polysilicates orpyrosilicates. The one or more cations make the compound electricallyneutral and are preferably selected from alkali metals, Mg, Be, Zn, Fe,Ca, Al, Mn or Zr, and/or mixtures thereof. Especially the alkali metalssuch as sodium, potassium and lithium are preferred. Particularlypreferred compounds herein are alkali metal orthosilicates such assodium or potassium orthosilicate, and alkali metal metasilicates suchas sodium or potassium metasilicate.

The aqueous solution may further contain a suitable amount of hydroxidesuch as sodium, potassium, and/or lithium hydroxide to raise the pHvalue. Said solution may further contain alkali earth metal salts or thefourth group (IVB) metal salts. The alkaline earth metal salts are, forexample, water soluble salts such as nitrates (strontium, magnesium, andbarium nitrate), sulfates, hydrochlorides, phosphates, acetates,oxalates and borates. The fourth group (IVB) metal salts are, forexample, titanium tetrachloride, titanium trichloride, titaniumpotassium fluoride, titanium potassium oxalate, titanium sulfate,titanium tetrachloride, zirconium chloroxide, zirconium dioxide,zirconium oxychloride, and zirconium tetrachloride etc. These alkalineearth metal salts and the fourth group (IVB) metal salts may be usedalone or in combination of more than two thereof. The aqueous solutioncomprising preferably has a concentration of 5-100 g/l, more preferablya concentration of 10-50 g/l and has a preferred pH value of 10-13 at25° C. The treatment is preferably performed by for example dipping thesupport in said aqueous solution at a preferred temperature of 20-100°C., and more preferably at 30-75° C. for preferably 0.5-40 s, and morepreferably for 1-20 s.

Alternatively, the aluminum oxide surface may be post-treated with aphosphate solution that may further contain an inorganic fluoride.Further, the aluminum oxide surface may be rinsed with an organic acidand/or salt thereof, e.g. carboxylic acids, hydrocarboxylic acids,sulphonic acids or phosphonic acids, or their salts, e.g. succinates,phosphates, phosphonates, sulphates, and sulphonates. A citric acid orcitrate solution is preferred. This treatment may be carried out at roomtemperature or may be carried out at a slightly elevated temperature ofabout 30° C. to 50° C. A further interesting treatment involves rinsingthe aluminum oxide surface with a bicarbonate solution. Still further,the aluminum oxide surface may be treated with polyvinylphosphonic acid,polyvinylmethylphosphonic acid, phosphoric acid esters of polyvinylalcohol, polyvinylsulfonic acid, polyvinylbenzenesulfonic acid, sulfuricacid esters of polyvinyl alcohol, and acetals of polyvinyl alcoholsformed by reaction with a sulfonated aliphatic aldehyde. It is furtherevident that one or more of these post treatments may be carried outalone or in combination. First treating with a silicate solutionfollowed by a treatment with polyvinylphosphonic acid,polyvinylmethylphosphonic acid, phosphoric acid esters of polyvinylalcohol, polyvinylsulfonic acid, polyvinylbenzenesulfonic acid, sulfuricacid esters of polyvinyl alcohol, and acetals of polyvinyl alcoholsformed by reaction with a sulfonated aliphatic aldehyde, is preferred.More detailed descriptions of these treatments are given in GB 1084070,DE 4423140, DE 4417907, EP 659909, EP 537633, DE 4001466, EP A 292801,EP A 291760 and U.S. Pat. No. 4,458,005.

More features of suitable supports, such as the preferred Ra (roughness)values of the grained surface, the anodic weight (g/m² of Al₂O₃ formedby the anodisation), and suitable post-anodic treatments are describedin EP 1 356 926A.

Coating Compositions

The lithographic printing plate precursor used in the present inventioncan be negative- or positive-working, i.e. can form ink-accepting areasat exposed or at non-exposed areas respectively. Below, suitableexamples of heat- and light-sensitive coatings are discussed in detail.

Heat-Sensitive Printing Plate Precursors.

The imaging mechanism of the heat-sensitive printing plate precursorscan be triggered by direct exposure to heat, e.g. by means of a thermalhead, or by the light absorption of one or more compounds in the coatingthat are capable of converting light, more preferably infrared light,into heat. These heat-sensitive lithographic printing plate precursorsare preferably not sensitive to visible light, i.e. no substantialeffect on the dissolution rate of the coating in the developer isinduced by exposure to visible light. Most preferably, the coating isnot sensitive to ambient daylight.

A first suitable example of a thermal printing plate precursor is aprecursor based on heat-induced coalescence of hydrophobic thermoplasticpolymer particles which are preferably dispersed in a hydrophilicbinder, as described in e.g. EP 770 494, EP 770 495, EP 770 497, EP 773112, EP 774 364, EP 849 090, EP 1 614 538, EP 1 614 539, EP 1 614 540,EP 1 777 067, EP 1 767 349, WO 2006/037716, WO 2006/133741 and WO2007/045515. According to a preferred embodiment, the thermoplasticpolymer particles include styrene and acrylonitrile units in a weightratio between 1:1 and 5:1 (styrene:acrylonitrile), e.g. in a 2:1 ratio.Examples of suitable hydrophilic binders are homopolymers and copolymersof vinyl alcohol, acrylamide, methylol acrylamide, methylolmethacrylamide, acrylic acid, methacrylic acid, hydroxyethyl acrylate,hydroxyethyl methacrylate and maleic anhydride/vinylmethylethercopolymers.

Preferably such a coating comprises an organic compound including atleast one phosphonic acid group or at least one phosphoric acid group ora salt thereof, as described in WO 2007/04551.

In a second suitable embodiment, the thermal printing plate precursorcomprises a coating comprising an aryldiazosulfonate homo- or copolymerwhich is hydrophilic and soluble in the processing liquid beforeexposure to heat or UV light and rendered hydrophobic and less solubleafter such exposure.

Preferred examples of such aryldiazosulfonate polymers are the compoundswhich can be prepared by homo- or copolymerization of aryldiazosulfonatemonomers with other aryldiazosulfonate monomers and/or with vinylmonomers such as (meth)acrylic acid or esters thereof, (meth)acrylamide,acrylonitrile, vinylacetate, vinylchloride, vinylidene chloride,styrene, α-methyl styrene etc. Suitable aryldiazosulfonate monomers aredisclosed in EP-A 339393, EP-A 507008 and EP-A 771645 and suitablearyldiazosulfonate polymers are disclosed in EP 507,008, EP 960,729, EP960,730 and EP1,267,211.

A highly preferred thermal printing plate precursor is positive-workingand includes a coating which is based on heat-induced solubilization ofan oleophilic resin. The oleophilic resin is preferably a polymer thatis soluble in an aqueous developer, more preferably an aqueous alkalinedevelopment solution with a pH between 7.5 and 14. Preferred polymersare phenolic resins e.g. novolac, resoles, polyvinyl phenols and carboxysubstituted polymers. Typical examples of these polymers are describedin DE-A-4007428, DE-A-4027301 and DE-A-4445820. The coating preferablycontains at least one layer which includes the phenolic resin(s). Thislayer is also referred to as “the imaging layer” or the first layer. Theamount of phenolic resin present in the imaging layer is preferably atleast 50% by weight, preferably at least 80% by weight relative to thetotal weight of all the components present in the imaging layer.

In a preferred embodiment, the oleophilic resin is a phenolic resinwherein the phenyl group or the hydroxy group is chemically modifiedwith an organic substituent. The phenolic resins which are chemicallymodified with an organic substituent may exhibit an increased chemicalresistance against printing chemicals such as fountain solutions orplate treating liquids such as plate cleaners. Examples of suchchemically modified phenolic resins are described in EP-A 0 934 822,EP-A 1 072 432, U.S. Pat. No. 5,641,608, EP-A 0 982 123, WO 99/01795,EP-A 02 102 446, EP-A 02 102 444, EP-A 02 102 445, EP-A 02 102 443, EP-A03 102 522. The modified resins described in EP-A 02 102 446, arepreferred, especially those resins wherein the phenyl-group of saidphenolic resin is substituted with a group having the structure —N═N-Q,wherein the —N═N— group is covalently bound to a carbon atom of thephenyl group and wherein Q is an aromatic group.

The oleophilic resin may also be mixed with or replaced by otherpolymers such as polymers including a urethane group and/or poly(vinylacetal) resins. Suitable poly(vinyl acetal) resins which are added inorder to improve the abrasion resistance of the coating are described inU.S. Pat. Nos. 5,262,270; 5,169,897; 5,534,381; 6,458,511; 6,541,181;6,087,066; 6,270,938; WO 2001/9682; EP 1 162 209; 6,596,460; 6,596,460;6,458,503; 6,783,913; 6,818,378; 6,596,456; WO 2002/73315; WO2002/96961; U.S. Pat. No. 6,818,378; WO 2003/79113; WO 2004/20484; WO2004/81662; EP 1 627 732; WO 2007/17162; WO 2008/103258; U.S. Pat. Nos.6,087,066; 6,255,033; WO 2009/5582; WO 2009/85093; WO 2001/09682; US2009/4599; WO 2009/99518; US 2006/130689; US 2003/166750; U.S. Pat. No.5,330,877; US 2004/81662; US 2005/3296; EP 1 627 732; WO 2007/3030; US2009/0291387; US 2010/47723 and US 2011/0059399.

The poly(vinyl acetal) resin preferably contains the following acetalmoiety:

wherein R¹ represents an aliphatic carbon chain such as a methyl, ethyl,propyl, butyl or pentyl group, an optionally substituted aryl group suchas a phenyl, benzyl, naphthyl, tolyl, ortho-meta- or para-xylyl,anthracenyl or phenanthrenyl, or an optionally substituted heteroarylgroup such as a pyridyl, pyrimidyl, pyrazoyl, triazinyl, imidazolyl,furyl, thienyl, isoxazolyl, thiazolyl and carbazoyl group. Mostpreferably the vinyl acetale is selected from vinyl formal, vinylethyral, vinyl propyral and/or vinyl butyral.

Preferred poly(vinyl acetal) resins are copolymers comprising acetalmoieties and ethylenic moieties as described in WO2014/106554,WO2015/158566, WO2015/173231, WO2015/189092 and WO2016/001023.Especially preferred poly(vinyl acetale) resins are resins includingethylenic moieties and acetal moieties including an optionallysubstituted aromatic or heteroaromatic group including at least onehydroxyl group (WO2014/106554), or poly(vinyl acetale) resins includingan optionally substituted aromatic or heteroaromatic group are resinsincluding at least one hydroxyl group in ortho or para position relativeto an electron withdrawing group (WO2015/158566).

The coating may further comprise a second layer that comprises one ormore other binder(s) which is insoluble in water and soluble in analkaline solution such as an organic polymer which has acidic groupswith a pKa of less than 13 to ensure that the layer is soluble or atleast swellable in aqueous alkaline developers. This layer is locatedbetween the layer described above comprising the oleophilic resin i.e.the imaging layer, and the hydrophilic support. This layer is alsoreferred to as “the second layer”. The binder may be selected from apolyester resin, a polyamide resin, an epoxy resin, an acrylic resin, amethacrylic resin, a styrene based resin, a polyurethane resin or apolyurea resin. The binder may have one or more functional groups. Thefunctional group(s) can be selected from the list of

(I) a sulfonamide group such as —NR—SO₂—, —SO₂—NR— or —SO₂—NR′R″ whereinR and R′ independently represent hydrogen or an optionally substitutedhydrocarbon group such as an optionally substituted alkyl, aryl orheteroaryl group; more details concerning these polymers can be found inEP 2 159 049;

(II) a sulfonamide group including an acid hydrogen atom such as—SO₂—NH—CO— or —SO₂—NH—SO₂— as for example disclosed in U.S. Pat. No.6,573,022 and/or EP 909 68(of 5)7; suitable examples of these compoundsinclude for example N-(p-toluenesulfonyl) methacrylamide andN-(p-toluenesulfonyl) acrylamide;

(III) an urea group such as —NH—CO—NH—, more details concerning thesepolymers can be found in WO 01/96119;

(IV) a star polymer in which at least three polymer chains are bonded toa core as described in EP 2 497 639;

(V) a carboxylic acid group;

(VI) a nitrile group;

(VII) a sulfonic acid group;

(VIII) a phosphoric acid group and/or

(IX) a urethane group.

(Co)polymers including a sulfonamide group are preferred. Sulfonamide(co)polymers are preferably high molecular weight compounds prepared byhomopolymerization of monomers containing at least one sulfonamide groupor by copolymerization of such monomers and other polymerizablemonomers. Preferably, in the embodiment where the poly(vinyl acetale)binder of the present invention is present in the second layer, thecopolymer comprising at least one sulfonamide group is present in thefirst layer located between the layer including the poly(vinyl acetale)binder of the present invention and the hydrophilic support.

Examples of monomers copolymerized with the monomers containing at leastone sulfonamide group include monomers as disclosed in EP 1 262 318, EP1 275 498, EP 909 657, EP 1 120 246, EP 894 622, U.S. Pat. No.5,141,838, EP 1 545 878 and EP 1 400 351. Monomers such as alkyl or aryl(meth)acrylate such as methyl (meth)acrylate, ethyl (meth)acrylate,butyl (meth)acrylate, benzyl (meth)acrylate, 2-phenylethyl(meth)acrylate, hydroxylethyl (meth)acrylate, phenyl (meth)acrylate;(meth)acrylic acid; (meth)acrylamide; a N-alkyl or N-aryl(meth)acrylamide such as N-methyl (meth)acrylamide, N-ethyl(meth)acrylamide, N-phenyl (meth)acrylamide, N-benzyl (meth)acrylamide,N-methylol (meth)acrylamide, N-(4-hydroxyphenyl)(meth)acrylamide,N-(4-methylpyridyl)(meth)acrylate; (meth)acrylonitrile; styrene; asubstituted styrene such as 2-, 3- or 4-hydroxy-styrene, 4-benzoicacid-styrene; a vinylpyridine such as 2-vinylpyridine, 3-vinylpyridine,4-vinylpyridine; a substituted vinylpyridine such as4-methyl-2-vinylpyridine; vinyl acetate, optionally the copolymerisedvinyl acetate monomeric units are at least partially hydrolysed, formingan alcohol group, and/or at least partially reacted by an aldehydecompound such as formaldehyde or butyraldehyde, forming an acetal orbutyral group; vinyl alcohol; vinyl acetal; vinyl butyral; a vinyl ethersuch as methyl vinyl ether; vinyl amide; a N-alkyl vinyl amide such asN-methyl vinyl amide, caprolactame, vinyl pyrrolydone; maleimide; aN-alkyl or N-aryl maleimide such as N-benzyl maleimide, are preferred.

Suitable examples of sulfonamide (co)polymers and/or their method ofpreparation are disclosed in EP 933 682, EP 982 123, EP 1 072 432, WO99/63407, EP 1 400 351 and EP 2 159 049. A highly preferred example of asulfonamide (co)polymer is described in EP 2 047 988 A in [0044] to[0046].

Specific preferred examples of sulphonamide (co)polymers are polymerscomprising N-(p-aminosulfonylphenyl) (meth)acrylamide,N-(m-aminosulfonylphenyl) (meth)acrylamide N-(o-aminosulfonylphenyl)(meth)acrylamide and or m-aminosulfonylphenyl (meth)acrylate.

(Co)polymers including an imide group are also preferred as a binder inthe heat-sensitive coating. Specific examples include derivatives ofmethyl vinyl ether/maleic anhydride copolymers and derivatives ofstyrene/maleic anhydride copolymers, that contain an N-substitutedcyclic imide monomeric units and/or N-substituted maleimides such as aN-phenylmaleimide monomeric unit and a N-benzyl-maleimide monomericunit. This copolymer is preferably alkali soluble. Suitable examples aredescribed in EP 933 682, EP 894 622 A [0010] to [0033], EP 901 902, EP 0982 123 A [007] to [0114], EP 1 072 432 A [0024] to [0043] and WO99/63407 (page 4 line 13 to page 9 line 37).

Polycondensates and polymers having free phenolic hydroxyl groups, asobtained, for example, by reacting phenol, resorcinol, a cresol, axylenol or a trimethylphenol with aldehydes, especially formaldehyde, orketones, may also be added to the heat-sensitive coating. Condensates ofsulfamoyl- or carbamoyl-substituted aromatics and aldehydes or ketonesare also suitable. Polymers of bismethylol-substituted ureas, vinylethers, vinyl alcohols, vinyl acetals or vinylamides and polymers ofphenylacrylates and copolymers of hydroxy-phenylmaleimides are likewisesuitable. Furthermore, polymers having units of vinylaromatics or aryl(meth)acrylates may be mentioned, it being possible for each of theseunits also to have one or more carboxyl groups, phenolic hydroxylgroups, sulfamoyl groups or carbamoyl groups. Specific examples includepolymers having units of 2-hydroxyphenyl (meth)acrylate, of4-hydroxystyrene or of hydroxyphenylmaleimide. The polymers mayadditionally contain units of other monomers which have no acidic units.Such units include vinylaromatics, methyl (meth)acrylate,phenyl(meth)acrylate, benzyl (meth)acrylate, methacrylamide oracrylonitrile.

The dissolution behavior of the coating can be fine-tuned by optionalsolubility regulating components. More particularly, developabilityenhancing compounds, development accelerators and development inhibitorscan be used. In the embodiment where the coating comprises more than onelayer, these ingredients can be added to the first layer and/or to thesecond layer and/or to an optional other layer of the coating.

Suitable developability enhancing compounds are (i) compounds which uponheating release gas as disclosed in WO 2003/79113, (ii) the compounds asdisclosed in WO 2004/81662, (iii) the compositions that comprises one ormore basic nitrogen-containing organic compounds as disclosed in WO2008/103258 and (iv) the organic compounds having at least one aminogroup and at least one carboxylic acid group as disclosed in WO2009/85093.

Examples of basic nitrogen-containing organic compounds useful in thedevelopability-enhancing compositions areN-(2-hydroxyethyl)-2-pyrrolidone, 1-(2-hydroxyethyl)piperazine,N-phenyldiethanolamine, triethanolamine,2-[bis(2-hydroxyethyl)amino]-2-hydroxymethyl-1.3-propanediol,N,N,N′,N′-tetrakis(2-hydroxyethyl)-ethylenediamine,N,N,N′,N′-tetrakis(2-hydroxypropyl)-ethylenediamine,3-[(2-hydroxyethyl)phenylamino]propionitrile, andhexahydro-1,3,5-tris(2-hydroxyethyl)-s-triazine. PreferablyN,N,N′,N′-tetrakis(2-hydroxypropyl)-ethylenediamine is used. Mixtures oftwo or more of these compounds are also useful. The basicnitrogen-containing organic compounds can be obtained from a number ofcommercial sources including BASF (Germany) and Aldrich Chemical Company(Milwaukee, Wis.).

The basic nitrogen-containing organic compound(s) is preferably presentin the coating in an amount of from 1 to 30% wt, and typically from 3 to15% wt, based on the total solids of the coating composition.

Preferably, one or more of the basic nitrogen-containing organiccompounds are used in combination with one or more acidicdevelopability-enhancing compounds, such as carboxylic acids or cyclicacid anhydrides, sulfonic acids, sulfinic acids, alkylsulfuric acids,phosphonic acids, phosphinic acids, phosphonic acid esters, phenols,sulfonamides, or sulfonimides, since such a combination may permitfurther improved developing latitude and printing durability.Representative examples of the acidic developability-enhancing compoundsare provided in [0030] to [0036] of US 2005/0214677. They may be presentin an amount of from 0.1 to 30% wt based on the total dry weight of thecoating composition. The molar ratio of one or more basicnitrogen-containing organic compounds to one or more acidicdevelopability-enhancing compounds is generally from 0.1:1 to 10:1 andmore typically from 0.5:1 to 2:1.

Development accelerators are compounds which act as dissolutionpromoters because they are capable of increasing the dissolution rate ofthe coating. For example, cyclic acid anhydrides, phenols or organicacids can be used in order to improve the aqueous developability.Examples of the cyclic acid anhydride include phthalic anhydride,tetrahydrophthalic anhydride, hexahydrophthalic anhydride,3,6-endoxy-4-tetrahydrophthalic anhydride, tetrachlorophthalicanhydride, maleic anhydride, chloromaleic anhydride, alpha-phenylmaleicanhydride, succinic anhydride, and pyromellitic anhydride, as describedin U.S. Pat. No. 4,115,128. Examples of the phenols include bisphenol A,p-nitrophenol, p-ethoxyphenol, 2,4,4′-trihydroxybenzophenone,2,3,4-trihydroxy-benzophenone, 4-hydroxybenzophenone,4,4′,4″-trihydroxy-triphenylmethane, and4,4′,3″,4″-tetrahydroxy-3,5,3′,5′-tetramethyltriphenyl-methane, and thelike. Examples of the organic acids include sulphonic acids, sulfinicacids, alkylsulfuric acids, phosphonic acids, phosphates, and carboxylicacids, as described in, for example, JP-A Nos. 60-88,942 and 2-96,755.Specific examples of these organic acids include p-toluenesulphonicacid, dodecylbenzenesulphonic acid, p-toluenesulfinic acid,ethylsulfuric acid, phenylphosphonic acid, phenylphosphinic acid, phenylphosphate, diphenyl phosphate, benzoic acid, isophthalic acid, adipicacid, p-toluic acid, 3,4-dimethoxybenzoic acid, 3,4,5-trimethoxybenzoicacid, 3,4,5-trimethoxycinnamic acid, phthalic acid, terephthalic acid,4-cyclohexene-1,2-dicarboxylic acid, erucic acid, lauric acid,n-undecanoic acid, and ascorbic acid. The amount of the cyclic acidanhydride, phenol, or organic acid contained in the coating ispreferably in the range of 0.05 to 20% by weight, relative to thecoating as a whole. Polymeric development accelerators such asphenolic-formaldehyde resins comprising at least 70 mol % meta-cresol asrecurring monomeric units are also suitable development accelerators.

In a preferred embodiment, the coating also contains developerresistance means, also called development inhibitors, i.e. one or moreingredients which are capable of delaying the dissolution of theunexposed areas during processing. The dissolution inhibiting effect ispreferably reversed by heating, so that the dissolution of the exposedareas is not substantially delayed and a large dissolution differentialbetween exposed and unexposed areas can thereby be obtained. Thecompounds described in e.g. EP 823 327 and WO 97/39894 act asdissolution inhibitors due to interaction, e.g. by hydrogen bridgeformation, with the alkali-soluble resin(s) in the coating. Inhibitorsof this type typically are organic compounds which include at least onearomatic group and a hydrogen bonding site such as a nitrogen atom whichmay be part of a heterocyclic ring or an amino substituent, an oniumgroup, a carbonyl, sulfinyl or sulfonyl group. Suitable dissolutioninhibitors of this type have been disclosed in e.g. EP 825 927 and EP823 327. Some of the compounds mentioned below, e.g. infrared dyes, suchas cyanines, and contrast dyes, such as quaternized triarylmethane dyes,can also act as a dissolution inhibitor.

Other suitable inhibitors improve the developer resistance because theydelay the penetration of the aqueous alkaline developer into thecoating. Such compounds can be present in the first layer and/or in theoptional second layer and/or in a development barrier layer on top ofsaid layer, as described in e.g. EP 864 420, EP 950 517, WO 99/21725 andWO 01/45958. The solubility and/or penetrability of the barrier layer inthe developer can be increased by exposure to heat and/or infraredlight.

Water-repellent polymers represent another type of suitable dissolutioninhibitors. Such polymers seem to increase the developer resistance ofthe coating by repelling the aqueous developer from the coating. In theembodiment where the coating comprises more than one layer, thewater-repellent polymers can be added to the first layer and/or to thesecond layer and/or in a separate layer provided on top of these layers.In the latter embodiment, the water-repellent polymer forms a barrierlayer which shields the coating from the developer and the solubility ofthe barrier layer in the developer or the penetrability of the barrierlayer by the developer can be increased by exposure to heat or infraredlight, as described in e.g. EP 864 420, EP 950 517 and WO99/21725.

Preferred examples of inhibitors which delay the penetration of theaqueous alkaline developer into the coating include water-repellentpolymers including siloxane and/or perfluoroalkyl units. Thepolysiloxane may be a linear, cyclic or complex cross-linked polymer orcopolymer. The term polysiloxane compound shall include any compoundwhich contains more than one siloxane group —Si(R,R′)—O—, wherein R andR′ are optionally substituted alkyl or aryl groups. Preferred siloxanesare phenylalkylsiloxanes and dialkylsiloxanes. The number of siloxanegroups in the polymer is at least 2, preferably at least 10, morepreferably at least 20. It may be less than 100, preferably less than60.

The water-repellent polymer may be a block-copolymer or agraft-copolymer including a polar block such as a poly- oroligo(alkylene oxide) and a hydrophobic block such as a long chainhydrocarbon group, a polysiloxane and/or a perfluorinated hydrocarbongroup. A typical example of a perfluorinated surfactant is Megafac F-177available from Dainippon Ink & Chemicals, Inc. Other suitable copolymerscomprise about 15 to 25 siloxane units and 50 to 70 alkyleneoxidegroups. Preferred examples include copolymers comprisingphenylmethylsiloxane and/or dimethylsiloxane as well as ethylene oxideand/or propylene oxide, such as Tego Glide 410, Tego Wet 265, TegoProtect 5001 or Silikophen P50/X, all commercially available from TegoChemie, Essen, Germany.

A suitable amount of such a water-repellent polymer in the coating isbetween 0.5 and 25 mg/m², preferably between 0.5 and 15 mg/m² and mostpreferably between 0.5 and 10 mg/m². When the water-repellent polymer isalso ink-repelling, e.g. in the case of polysiloxanes, higher amountsthan 25 mg/m² can result in poor ink-acceptance of the non-exposedareas. An amount lower than 0.5 mg/m² on the other hand may lead to anunsatisfactory development resistance.

It is believed that during coating and drying, the water-repellentpolymer or copolymer acts as a surfactant and tends to position itself,due to its bifunctional structure, at the interface between the coatingand air and thereby forms a separate top layer, even when applied as aningredient of the coating solution. Simultaneously, such surfactantsalso act as spreading agents which improve the coating quality.Alternatively, the water-repellent polymer or copolymer can be appliedin a separate solution, coated on top of the coating including one oroptional more layers. In that embodiment, it may be advantageous to usea solvent in the separate solution that is not capable of dissolving theingredients present in the other layers so that a highly concentratedwater-repellent phase is obtained at the top of the coating.

The coating of the heat-sensitive printing plate precursors describedabove preferably also contains an infrared light absorbing dye orpigment which, in the embodiment where the coating comprises more thanone layer, may be present in the first layer, and/or in the secondlayer, and/or in an optional other layer. Preferred IR absorbing dyesare cyanine dyes, merocyanine dyes, indoaniline dyes, oxonol dyes,pyrilium dyes and squarilium dyes. Examples of suitable IR dyes aredescribed in e.g. EP-As 823327, 978376, 1029667, 1053868, 1093934; WO97/39894 and 00/29214. Preferred compounds are the following cyaninedyes:

The concentration of the IR-dye in the coating is preferably between0.25 and 15.0% wt, more preferably between 0.5 and 10.0% wt, mostpreferably between 1.0 and 7.5% wt relative to the coating as a whole.

The coating may further comprise one or more colorant(s) such as dyes orpigments which provide a visible color to the coating and which remainin the coating at the image areas which are not removed during theprocessing step. Thereby a visible image is formed and examination ofthe lithographic image on the developed printing plate becomes feasible.Such dyes are often called contrast dyes or indicator dyes. Preferably,the dye has a blue color and an absorption maximum in the wavelengthrange between 600 nm and 750 nm. Typical examples of such contrast dyesare the amino-substituted tri- or diarylmethane dyes, e.g. crystalviolet, methyl violet, victoria pure blue, flexoblau 630, basonylblau640, auramine and malachite green. Also the dyes which are discussed indepth in EP-A 400,706 are suitable contrast dyes. Dyes which, combinedwith specific additives, only slightly color the coating but whichbecome intensively colored after exposure, as described in for exampleWO2006/005688 may also be used as colorants.

Optionally, the coating may further contain additional ingredients suchas surfactants, especially perfluoro surfactants, silicon or titaniumdioxide particles or polymers particles such as matting agents andspacers.

Any coating method can be used for applying one or more coatingsolutions to the hydrophilic surface of the support. The multi-layercoating can be applied by coating/drying each layer consecutively or bythe simultaneous coating of several coating solutions at once. In thedrying step, the volatile solvents are removed from the coating untilthe coating is self-supporting and dry to the touch. However it is notnecessary (and may not even be possible) to remove all the solvent inthe drying step. Indeed the residual solvent content may be regarded asan additional composition variable by means of which the composition maybe optimized. Drying is typically carried out by blowing hot air ontothe coating, typically at a temperature of at least 70° C., suitably80-150° C. and especially 90-140° C. Also infrared lamps can be used.The drying time may typically be 15-600 seconds.

Between coating and drying, or after the drying step, a heat treatmentand subsequent cooling may provide additional benefits, as described inWO99/21715, EP-A 1074386, EP-A 1074889, WO00/29214, and WO/04030923,WO/04030924, WO/04030925.

The printing plate precursor can be exposed to infrared light by meansof e.g. LEDs or a laser. Most preferably, the light used for theexposure is a laser emitting near infrared light having a wavelength inthe range from about 750 to about 1500 nm, more preferably 750 to 1100nm, such as a semiconductor laser diode, a Nd:YAG or a Nd:YLF laser. Therequired laser power depends on the sensitivity of the plate precursor,the pixel dwell time of the laser beam, which is determined by the spotdiameter (typical value of modern plate-setters at 1/e² of maximumintensity: 5-25 μm), the scan speed and the resolution of the exposureapparatus (i.e. the number of addressable pixels per unit of lineardistance, often expressed in dots per inch or dpi; typical value:1000-4000 dpi).

Light-Sensitive Printing Plate Precursors.

In addition to the above thermal materials, also light-sensitivecoatings can be used. Typical examples of such plates are theUV-sensitive “PS” plates and the so-called photopolymer plates whichcontain a photopolymerizable composition that hardens upon exposure tolight.

In a particular embodiment of the present invention, a conventional,UV-sensitive “PS” plate precursor is used. Suitable examples of suchplates precursors, that are sensitive in the range of 300-450 nm (nearUV and blue light), have been discussed in EP 1,029,668 A2. Positive-and negative-working compositions are typically used in “PS” plateprecursors.

The positive-working imaging layer preferably comprises ano-naphtoquinonediazide compound (NQD) and an alkali soluble resin.Particularly preferred are o-naphthoquinone-diazidosulphonic acid estersor o-naphthoquinone diazidocarboxylic acid esters of various hydroxylcompounds and o-naphthoquinone-diazidosulphonic acid amides oro-naphthoquinone-diazidocarboxylic acid amides of various aromatic aminecompounds. Two variants of NQD systems can be used: one-componentsystems and two-component systems. Such light-sensitive printing plateshave been widely disclosed in the prior art, for example in U.S. Pat.No. 3,635,709, J.P. KOKAI No. 55-76346, J.P. KOKAI No. Sho 50-117503,J.P. KOKAI No. Sho 50-113305, U.S. Pat. Nos. 3,859,099; 3,759,711; GB-A739654, U.S. Pat. No. 4,266,001 and J.P. KOKAI No. 55-57841.

The negative-working layer of a “PS” plate preferably comprises adiazonium salt, a diazonium resin or an aryldiazosulfonate homo- orcopolymer. Suitable examples of low-molecular weight diazonium saltsinclude: benzidine tetrazoniumchloride, 3,3′-dimethylbenzidinetetrazoniumchloride, 3,3′-dimethoxybenzidine tetrazoniumchloride,4,4′-diaminodiphenylamine tetrazoniumchloride, 3,3′-diethylbenzidinetetrazoniumsulfate, 4-aminodiphenylamine diazoniumsulfate,4-aminodiphenylamine diazoniumchloride, 4-piperidino anilinediazoniumsulfate, 4-diethylamino aniline diazoniumsulfate and oligomericcondensation products of diazodiphenylamine and formaldehyde. Examplesof diazo resins include condensation products of an aromatic diazoniumsalt as the light-sensitive substance. Such condensation products aredescribed, for example, in DE-P-1 214 086. The light- or heat-sensitivelayer preferably also contains a binder e.g. polyvinyl alcohol.

Upon exposure the diazo resins or diazonium salts are converted fromwater soluble to water insoluble (due to the destruction of thediazonium groups) and additionally the photolysis products of the diazomay increase the level of crosslinking of the polymeric binder or diazoresin, thereby selectively converting the coating, in an image pattern,from water soluble to water insoluble. The unexposed areas remainunchanged, i.e. water-soluble.

Such printing plate precursors can be developed using an aqueousalkaline solution as described above.

In a second suitable embodiment, the light sensitive printing plateprecursor is based on a photo-polymerisation reaction and contains acoating comprising a photocurable composition comprising a free radicalinitiator (as disclosed in for example U.S. Pat. Nos. 5,955,238;6,037,098; 5,629,354; 6,232,038; 6,218,076; 5,955,238; 6,037,098;6,010,824; 5,629,354; DE 1,470,154; EP 024,629; EP 107,792; U.S. Pat.No. 4,410,621; EP 215,453; DE 3,211,312 and EP A 1,091,247) apolymerizable compound (as disclosed in EP 1 161 4541, EP 1 349 006,WO2005/109103, EP 1 788 448, EP 1 788 435, EP 1 788 443, EP 1 788 434)and a polymeric binder (as disclosed in for example US2004/0260050,US2005/0003285; US2005/0123853; EP 1,369,232; EP 1,369,231; EP1,341,040; US 2003/0124460, EP 1 241 002, EP 1 288 720, U.S. Pat. Nos.6,027,857, 6,171,735; 6,420,089; EP 152,819; EP 1,043, 627; U.S. Pat.No. 6,899,994; US2004/0260050; US 2005/0003285; US2005/0170286;US2005/0123853; US2004/0260050; US2005/0003285; US 2004/0260050; US2005/0003285; US 2005/0123853 and US2005/0123853). Other ingredientssuch as sensitizers, co-initiators, inhibitors, adhesion promotingcompounds, colorants, surfactants and/or printing out agents mayoptionally be added. These printing plate precursors can be sensitizedwith blue, green or red light (i.e. wavelength range between 450 and 750nm), with violet light (i.e. wavelength range between 350 and 450 nm) orwith infrared light (i.e. wavelength range between 750 and 1500 nm)using for example an Ar laser (488 nm) or a FD-YAG laser (532 nm), asemiconductor laser InGaN (350 to 450 nm), an infrared laser diode (830nm) or a Nd-YAG laser (1064 nm).

To protect the surface of the coating of the heat and/or light sensitiveprinting plate precursors, in particular from mechanical damage, aprotective layer may also optionally be applied. The protective layergenerally comprises at least one water-soluble binder, such as polyvinylalcohol, polyvinylpyrrolidone, partially hydrolyzed polyvinyl acetates,gelatin, carbohydrates or hydroxyethylcellulose, and can be produced inany known manner such as from an aqueous solution or dispersion whichmay, if required, contain small amounts—i.e. less than 5% by weightbased on the total weight of the coating solvents for the protectivelayer—of organic solvents. The thickness of the protective layer cansuitably be any amount, advantageously up to 5.0 μm, preferably from 0.1to 3.0 μm, particularly preferably from 0.15 to 1.0 μm.

Optionally, the coating may further contain additional ingredients suchas surfactants, especially perfluoro surfactants, silicon or titaniumdioxide particles, organic or inorganic spacer particles or mattingagents.

Any coating method can be used for applying two or more coatingsolutions to the hydrophilic surface of the support. The multi-layercoating can be applied by coating/drying each layer consecutively or bythe simultaneous coating of several coating solutions at once. In thedrying step, the volatile solvents are removed from the coating untilthe coating is self-supporting and dry to the touch. However it is notnecessary (and may not even be possible) to remove all the solvent inthe drying step. Indeed the residual solvent content may be regarded asan additional composition variable by means of which the composition maybe optimized. Drying is typically carried out by blowing hot air ontothe coating, typically at a temperature of at least 70° C., suitably80-150° C. and especially 90-140° C. Also infrared lamps can be used.The drying time may typically be 15-600 seconds.

Between coating and drying, or after the drying step, a heat treatmentand subsequent cooling may provide additional benefits, as described inWO99/21715, EP-A 1074386, EP-A 1074889, WO00/29214, and WO/04030923,WO/04030924, WO/04030925.

EXAMPLES Example 1 Exposure

Energy Elite Eco printing plate precursors, commercially available fromAGFA Graphics NV, were exposed at different energy densities on a CreoTrendsetter 3244, commercially available from Kodak, with a 20 W imaginghead and operating at 140 rpm and 2400 dpi.

The Energy Elite Eco printing plate precursors were subsequently fullyimaged at the defined plate sensitivity (see below), except for a twosquared centimeter solid area which was kept on the plate to be able tocontrol printing conditions.

Development

After exposure, the printing plate precursors were inserted in a SXprocessor commercially available from Agfa Graphics NV, filled withTHD300 developer and Unifin gum, both solutions commercially availablefrom AGFA Graphics NV. The development conditions were set at atemperature of 22° C. and a dwell time of 18 s for all examples.

Plate Sensitivity

The plate sensitivity of the processed plates was determined and definedas the energy density at which the 1×1 pixel checkerboard pattern has a52% dot area coverage as measured with a GretagMacbeth D19Cdensitometer, commercially available from GretagMacbeth AG.

Oxidation Spot Evaluation

The printing plates thus obtained were subjected to different chemicalsolutions, as described in Table 1, with the use of a cotton padimpregnated with said solutions.

The plates were subsequently dried at room temperature for 1 h andplaced in a conditioned room for 3 days at 20° C. and 85% relativehumidity (RH).

The thus obtained plates were subsequently mounted on a Ryobi 522 HXX™sheetfet press. Printing was performed on offset maxi 70 g paper,commercially available from Igepa, using Flint K+E 800 Skinnex Black asink, commercially available from Flint, and 3% Prima FS404AS,commercially available from Agfa Graphics N.V.+5% isopropyl alcohol asfountain solution. The press was set to obtain an optical density of1.25 when measured on the solid area as measured with a GretagMacbethD19C densitometer, commercially available from GretagMacbeth AG. 250sheets were then printed.

Oxidation spots which may be present on the plate, i.e. areas which areink accepting, result in black spots on print. These black spots werevisually evaluated on the 250th sheet in the area of the plate which wastreated with the chemical solution. The amount of black spots werevisually evaluated and a score from 0 to 3 was given:

0. no black spots visible;1. a few scattered black spots visible;2. many black spots visible;3. area full of black spots.

The results are summarized in Table 1.

TABLE 1 Black spots evaluation. Chemical Cl⁻** NO₃ ⁻*** Black Testsolutions* ppm ppm spots**** 1 CS0 — — 0 2 CS1 200 — 2 3 CS2 650 — 3 4CS3 650  500 1 5 CS4 650 2500 0 6 CS5 650 5000 0 *Solutions preparedwith demineralized water; **chlorine was added in the form of NaCl;***nitrate was added in the form of NaNO₃; ****visual scale used: 0. noblack spots visible; 1. a few scattered black spots visible; 2. manyblack spots visible; 3. area full of black spots.

The results in Table 1 show that:

-   -   Black spots on the 250 th printed sheet are visible after        treating the plate with a solution including chlorine ions        (Tests 1 to 3);    -   No or only a few scattered black spots are visible on the 250 th        printed sheet after treating the plate with a solution including        both chlorine ions and nitrate ions (Tests 4 to 6);

In conclusion, nitrate ions prevent the formation of oxidation spotsvisible in the form of black spots on the printed sheets.

Example 2 Exposure

Energy Elite Eco printing plate precursors, commercially available fromAGFA Graphics NV were exposed as described in Example 1.

Development

After exposure, the printing plate precursors were inserted in a SXprocessor commercially available from Agfa Graphics NV, filled withTHD300 developer commercially available from AGFA Graphics NV, and thegum solutions as defined in Table 2. The development conditions were setat a temperature of 22° C. and a dwell time of 18 s for all examples.

Oxidation Spot Evaluation

The thus obtained printing plates were subjected to the chemicalsolution CS2, as described in Table 1, with the use of a cotton padimpregnated with said solution.

The plates were subsequently dried at room temperature for 1 h andplaced in a conditioned room for 3 days at 20° C. and 85% relativehumidity (RH).

The printing plates were subsequently mounted on a Ryobi 522 HXX™sheetfet press and printing was performed as described in Example 1.

Evaluation of oxidation spots on the plate was carried out as describedin Example 1. The results of the occurrence of black spots aresummarized in Table 2.

TABLE 2 Black spot evaluation. Test Gum solutions Black spots*** 7Unifin* 3 8 Unifin* + 500 ppm of NO₃ ⁻** 0 *commercially available fromAgfa Graphics NV; **nitrate salt was added in the form of NaNO₃;***visual scale used: 0. no black spots visible; 1. a few scatteredblack spots visible; 2. many black spots visible; 3. area full of blackspots.

The results in Table 2 show that:

-   -   many scattered black spots are visible after treating the plate        with Unifin gum solution on the 250th sheet (Test 7);    -   no or only a few scattered black spots are visible after        treating the plate with Unifin gum solution including 500 ppm of        nitrate ions (Test 8).

In conclusion, nitrate ions prevent the formation of oxidation spotsvisible in the form of black spots on the printed sheets.

1-15. (canceled)
 16. A method for processing a lithographic printingplate material, the method comprising the steps of: developing anexposed precursor of the plate material with an alkaline developmentsolution to remove non-image areas; and treating the developed precursorconsecutively with a first gum solution and a second gum solution suchthat the second gum solution overflows into the first gum solution;wherein the first gum solution and/or the second gum solution includesat least one nitrate salt.
 17. The method according to claim 16, whereinthe at least one nitrate salt includes sodium nitrate or potassiumnitrate.
 18. The method according to claim 16, wherein the alkalinedevelopment solution is substantially silicate-free.
 19. The methodaccording to claim 16, wherein the plate material includes an aluminumsupport that has been grained, anodized, and treated with an aqueoussolution including a compound containing a silicate anion and one ormore cations.
 20. The method according to claim 19, wherein the silicateanion is selected from the group of phosphosilicate, orthosilicate,metasilicate, hydrosilicate, polysilicate, and pyrosilicate.
 21. Themethod according to claim 19, wherein the silicate anion includes anorthosilicate or a metasilicate.
 22. The method according to claim 18,wherein the substantially silicate-free alkaline development solution isan aqueous solution which has a pH above 10 and includes ammoniumhydroxide, sodium hydroxide, lithium hydroxide, potassium hydroxide,and/or organic amines, and/or mixtures thereof.
 23. The method accordingto claim 18, wherein the silicate-free alkaline development solutionincludes a non-reducing sugar and a base.
 24. The method according toclaim 23, wherein the non-reducing sugar includes a saccharide which isfree of free aldehyde groups or ketone groups.
 25. The method accordingto claim 16, wherein the method comprises no rinsing step between thestep of developing the exposed precursor and the step of treating thedeveloped precursor.
 26. The method according to claim 16, wherein thefirst gum solution and the second gum solution each have a pH value from0 to
 8. 27. The method according to claim 16, wherein the plate materialis not brushed during the step of developing the exposed precursor withthe alkaline development solution.
 28. The method according to claim 16,wherein the plate material includes a heat-sensitive, positive-workingcoating on a support, and the coating includes an image recording layerincluding an infrared absorbing agent and an oleophilic resin.
 29. Themethod according claim 16, wherein the plate material includes aheat-sensitive positive-working coating on a support, and the coatingincludes: a first layer including an oleophilic resin and/or a vinylacetal (co-)polymer; and a second layer located between the first layerand the support, the second including at least one polymer that includesa sulfonamide group, an imide group, a nitrile group, a urethane group,a urea group, a carboxyl group, a sulfonic acid group, and/or aphosphoric acid group.
 30. A method for processing a plurality oflithographic printing plate materials with a processing apparatus, themethod comprising the steps of: filling a development unit of theprocessing apparatus with a development solution; repeating the stepsaccording to claim 16 so as to process the plurality of printing platematerials; and draining the development solution from the developmentunit.